Rotational Inertia

Corla Jean Wilson-Hawkins Bethune Elementary School
3030 West Arthington
Chicago, Illinois 60612


To demonstrate how the resistance of an object to rotation is rotational

Materials Needed:

This list is for a class size of 32 students divided into groups.

4 ramps 4 meter sticks
4 cans of broth soup covered with paper 8 100 gm weights
4 cans of tomato soup covered with paper masking tape
pendulum balance beam and long pole
hollow wheel solid wheel
turn table for turning (if possible - optional)


To begin the lesson, have the students give you several brainstorming
definitions of what they think inertia is. Next discuss what they think
rotational inertia is and write that on the board. After discussing the
vocabulary, have one student from each group sit on the turn table. Spin them
slowly. Using equal weights or books in their hands have them pull their hands
close to their bodies and then extend them away from their body. As they pull
their hands closer to their bodies they will find that they spin faster than
they do when their hands are extended outwards. Use this opportunity to discuss
how the distribution of mass makes a difference in the rotational inertia.

Next, using your groups, take the 4 meter sticks and tape the 100 gram weights
to the bottom of the stick. Put one on each side and tape around it. Have
students collect data on who could balance it vertically the longest with the
weights on the bottom and again with the weights on the top. Have the students
explain why they thought it was easier to balance the weight on the top of the
stick versus the bottom. Some will be good both ways but the norm tends to be
with the weights on the top.

Using the ramps, have your groups take the soup cans of different contents that
are covered with paper and time them as they roll down the ramp. Have them make
a graph charting the time it took and the can that won. Have them to try to
figure out why one can won over the other. After a thorough discussion, uncover
the cans and talk about the contents in the cans and the distribution of the

Now take the balance beam and have it properly mounted with the wooden stands on
each end. Have a student walk on the beam with their hands in their pockets,
and again with their hands extended holding a long pole or long stick. Talk
about which way was easier to walk. Again reinforce that it is easier to
balance when the rotational inertia is farther away from the axis.

For some additional fun, make a pendulum and push it back and forth on a long
string. Ask the children to tell you how it is moving. Then shorten the string
and let them tell you how it is going. Of course the shorter one moves faster
because it is closer to the axis. At this point the children can make a
pendulum for a hands on activity.

For a final activity take a ring and a solid disk and roll them down a ramp.
They do not have to be the same weight or size because the theory will still
prove itself. Have the children guess which one will come down first. Explain
to them that since the wooden disk is solid, it's mass is closer to the axis
than the hollow ring which has all it's mass on the outer rim.


The conclusion of this lesson would be to have students explain what we did
today in all our demonstrations. Ask them to tell you what they liked best and
what they liked the least. See if any of your students can think of other
examples of rotational inertia that were not used today that can be added to the


Often times when we do a lot of hands on activities, we fail to reinforce the
comprehension version in writing. My evaluation of this process will be a short
quiz that would include all the examples that we had today and to have the
children explain them to me. I would also have them draw some of the demos for
me on paper so that I know who really was aware of what was happening in the
demonstration. I would be looking for 90% accuracy on the quiz.


Event Results

bottles empty-easy to move full-hard to move
turntable mass at center-easy to rotate mass at edge-harder to do
meter stick wts on bottom hard to balance wts on top easy to balance
balance beam mass at center-easy to fall mass out-easier to stay
pendulums short-swings faster long-swings slower
soup cans tomato solid-rolls fast chicken-liquid rolls slower
disk and ring disk is faster-mass spread ring is slower-mass all at
uniformly throughout the edge (further out)


Conceptual Physics - Addison Wesley Paul G. Hewitt 1986 pg 192-204

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